1. Field of the Invention
The present invention generally relates to systems adapted to enable determining the position of objects in space, for example, but not limitatively, the objects' location on the Earth's surface, particularly to Global Navigation Satellite Systems (GNSSs) like the Global Positioning System (GPS). More specifically, the invention concerns a method for the acquisition of signals used for determining an object's location.
2. Description of the Related Art
GNSSs, and, particularly, the GPS are systems based on signal transmitters transported by a constellation of satellite vehicles orbiting around the Earth, and designed to enable the determination of the position of objects on the Earth surface.
Consumer electronic devices exploiting the GPS for purposes of location determination and aid to navigation are nowadays very common.
In general terms, in the GPS system a receiver (e.g., part of a navigation tool, a mobile phone, a Personal Digital Assistant-PDA-, a palmtop computer) acquires signals transmitted by transmitters on four or more satellite vehicles, to derive a three-dimensional location of the object and a current time stamp.
The GPS receiver normally receives the signals transmitted by multiple satellite transmitters, and performs an operation referred to as “signal acquisition”, which in general terms is a coarse synchronization process by which the receiver estimates from which transmitters, i.e. by which satellite vehicles the received signals have been transmitted. The information obtained by performing the signal acquisition operation is then exploited in a following signal tracking process, that enables the receiver to track the acquired signal sources.
Each GPS transmitter on a satellite vehicle emits a radio carrier signal at microwave frequency, referred to as the “L1” frequency.
The carrier signal at the L1 frequency (1575.42 GHz), which is exploited by commercial positioning and navigation equipment, is modulated by a Coarse Acquisition (C/A) code, a Precise (P) acquisition code, and by data that make up a “navigation message”.
The C/A code is a Pseudo Random Noise (PRN) code comprised of 1023 bits (also referred to as “chips”) that is repeated every millisecond (i.e., at a nominal repetition frequency of 1 KHz). All the satellite transmitters transmit over the same radio carrier at the L1 frequency, according to a CDMA (Code Division Multiple Access) multiplexing scheme which uses the C/A codes as spreading codes (Gold codes). Each satellite transmitter is assigned a unique C/A code, different from the C/A codes assigned to the other transmitters. The C/A codes assigned to the transmitters on the constellation of satellite vehicles are freely available to the public.
The P code is repeated at a higher frequency, and thus allows a more precise determination of the receiver's location; however, the P code is encrypted, and not available to the public.
The data making up the navigation message form a 50 Hz digital signal, used to modulate the phase (phase modulation) of the radio carrier, and consisting of data bits that encode a time stamp (the so-called “GPSToW”, or “GPS Time of the Week”, the GPS satellite vehicle's orbit parameters (including the so-called “almanac” and “ephemeris”), clock corrections, and other parameters; all these data are useful for the receiver to calculate and update its location on the Earth surface.
One approach to the signal acquisition process calls for the receiver to generate a local signal, modulated by a local replica of the PRN code that is broadcast by the transmitter on the satellite vehicle(s) potentially visible overhead the receiver, and determining a correlation between the received signal and the locally-generated signal. The correlation is typically performed by calculating both the “In phase” (I) and the “Quadrature” (Q) correlation integrals.
The results of the correlation integrals are strongly affected by the data bits of the navigation message that modulate the phase of the transmitted radio carrier signal; these data bits are unknown at a standard standalone receiver. As a consequence, the results of the I and Q correlation integrals are not reliable when the integration time is extended beyond the duration (20 ms) of one data bit of the navigation message, due to the possible (unknown) phase inversions experienced by the received signal.
A problem arises when the received signal is degraded, like for example when the receiver is inside a building (indoor environment): in these cases, to compensate for the signal degradation, the integration time, i.e., the duration of the received signal to be considered for performing the signal acquisition (and for the signal tracking after the signal has been acquired), should be extended to some seconds, i.e. well beyond the duration of one data bit; the correlation between the received signal and the locally-generated signal is thus calculated over a time interval that spans multiple navigation message data bits, and it is not possible to neglect the phase transitions (changes in sign) of the radio carrier signal.
In U.S. Pat. No. 6,933,886 a signal acquisition technique using data bit information is disclosed, wherein coherent correlation (the calculation of I and Q correlation integrals for sampled data that is associated with the received signal) may be performed by breaking the signal into small blocks of 1 ms length, and performing calculations on a block-by-block basis. In particular, using data bit information, i.e. information about the data message being transmitted by the signal source, the I and Q correlation integrals are calculated for each small block of data. Due to the fact that the time may be known only approximately, the blocks may not be able to be classified with certainty as to which one corresponds to which position within the 20 milliseconds time span before potential sign changes or flips. Thus, for those blocks of data for which a flip may occur, the I and Q correlation integrals are calculated both with and without a change in sign. In other words, for those blocks of data near the 20 milliseconds marks, the I and Q correlation integrals are calculated both with and without a change in sign.